This invention relates to the molding of plastic closures and more particularly to the molding of plastic closures having relatively large center panels. However, the invention is not necessarily so limited.
This invention is especially applicable to relatively large closures such as No. 403 and No. 502 closures and reclosures that are used in the packaging of margarine, as reclosures for coffee cans, and for other purposes. Although the invention is applicable to closures of various sizes, it is primarily intended for use with closures having a wall thickness that is small in relation to the surface area of the closure and the closures made from quite flexible thermoplastic materials such as low density polyethylene.
Conventional closures used in the packaging of margarine or the like have a large center panel of substantially uniform thickness, the outer margins of which join with an outer top panel part and a skirt which is usually molded thicker than, and thus stiffer than, the center panel. Many millions of such closures are produced and used every year.
Although the success of the conventional closures cannot be questioned, it would obviously be desirable to reduce the amount of plastic used in the production of each closure since any savings would be multiplied millions of times over each year.
Furthermore the conventional closures suffer from a drawback in that it is practically impossible to mold the closures without some warping of the closure rims. The degree of warping is variable, but the warpage is often sufficiently great that it can cause difficulty in the handling of molded closures up to the time they are placed on the containers. Occasionally the warpage may be so great as to make the closure appear to be an inferior product.
Conventional closures are typically molded with the use of injection molding machines provided with tooling having a pin hole gate opening to the center of the closure. The mold cavity is so shaped that the plastic in a molten state flowing through the gate flows into the cavity and throughout the cavity in an ever-expanding circle until the cavity is full. Thus at every point the plastic flows radially outwardly from the gate. After the cavity is filled, the plastic is permitted to cure to a solid state, which takes just a few seconds, and the complete part is ejected from the mold. Heretofore it has not been considered practicable to injection mold extremely thin walled closures. The molten plastic is injected under high pressure to fill a mold cavity. Injection pressures increase as wall thicknesses decrease because of the pressure required to cause the plastic to flow radially outwardly from the gate in very thin sections. The problem of filling very thin sections is compounded by the fact that the plastic begins to cure and thus become more viscous at a relatively rapid rate so that, when filling a cavity that forms thin sections following conventional practice, the radially outermost portions of the plastic begin to set up and thus further increase the required injection pressure.